Leishmaniasis, a tropical neglected disease, is a vector-borne disease caused by protozoan parasites of genus Leishmania. The parasites are carried only by female sandflies as the female flies need blood for the development of eggs. They become infected with the Leishmania parasites while they suck blood from an infected person or animal and then transmit it to another host. Of 900 species of the sand fly, over 70 species are found to be associated with the transmission of leishmaniasis. One of the major vectors for leishmaniasis is the Phlebotomine sandfly. There are around 500 known Phlebotomine species, however, nearly 30 species are capable to transmit leishmaniasis ( https://www.who.int/leishmaniasis/en/).
In order to be incriminating natural vectors, they should have the following criteria. The wild females not having recent blood meal (<36 hrs) should contain promastigotes of Leishmania parasites. The anterior midgut of infected females sand flies must have infective forms of Leishmania. The flies should be attracted to and bite humans and other reservoir hosts. They have to be strongly associated with humans and any reservoir host and finally, the experimental transmission is achieved after infection from natural host species or equivalent laboratory model. To date, no published data have been documented to verify the vectors found in Thailand to prove as incriminating natural vectors according to the criteria mentioned above. Few studies have reported the presence of Leishmania DNA in female sandflies collected from different provinces of Thailand by using molecular tools. Similarly, few investigators have studied the natural habitats of the sandflies and their related host. On the basis of these characteristics, they have suspected these vectors as the potential natural vectors of leishmaniasis.
Chamnarn and team had collected 2401 Phlebotomine sand flies from 16 limestone caves (temperature range 26-28°C) in Kanchanaburi province, Thailand, and identified them following standard protocol. The study had updated to a total number of 26 species of sandflies belonged to the four genera Sergentomyia, Phlebotomus, Nemopalpus, and Chinius in Thailand. These are C. barbazani, N. vietnamensis, P. asperulus, P. barguesae, P. betisi, P. hoepplii, P. major major, P. mascomai, P. philippinensis gouldi, P. pholetor, P. stantoni, P. teshi, S. anodontis, S. bailyi, S. barraudi, S. brevicaulis, S. dentata, S. gemmea, S. hodgsoni hodgsoni, S. indica, S. iyengari, S. perturbans, S. phasukae, S. punjabensis, S. quatei and S. sylvatica. The most frequent cave species found in this study were P. major major and S. anodontis. The human biting species, P. major was also the first time reported from this study. They have observed ecological habitats and behaviors (host feeding, biting activity, resting areas at day time, sheltering places at night) of sandflies to identify. However, they are not concerned about whether there was a presence of Leishmaniainfective forms in their midguts or not. They only proposed the sand flies as potential natural vectors for leishmaniasis (Apiwathnasorn et al., 2011). Late one more species, S. mahadevani had been identified, and altogether 27 species have been reported date in Thailand to date.
Among these potential vectors, different studies have confirmed different species that transmit the Leishmania parasites. Kanjanopas and team collected sandflies from individual households in Hat Samran District, Trang Province, southern Thailand where coinfection of visceral leishmaniasis and Human Immunodeficiency Syndrome (HIV) had been reported. They identified the female sandflies with the help of Entomologists and finally sent them in Molecular laboratory in Taiwan. They have evaluated for natural infections with L. siamensis confirmed by amplifying heat shock protein 70 (hsp70) of Leishmania parasite by PCR method. Although other criteria had not been studied, S. (Neophlebotomus)gemmea is considered as a potential natural vector for L. siamensis (Kanjanopas et al., 2013).
Chusri et al. carried out active human case surveys processing blood, saliva and urine samples from 99 villagers living in an affected area in Na Thawi District. The team had also studied details of animal reservoirs including blood samples from dogs, cats, black rats, and Indochinese ground squirrels. Sandflies were collected from villagers’ houses and plantation which were identified at office of Disease Prevention and Control. The presence of Leishmania parasite in those sandflies were confirmed by amplifying parasite specific 18s rRNA followed by nucleotide sequencing. The study finally reported female S. (Neophlebotomus) gemmea and female S. (Parrotomyia) barraudi were potential natural vectors for L. siamensis (Chusri et al., 2014).
Another study by Sukra and the team had reported six sand fly vectors of Sergentomyia; S. gemmea, S. iyengari, S. barraudi, S. indica, S. silvatica and S. perturbans as potential natural vectors of leishmaniasis. The team had collected sandflies from three provinces of Thailand; Phang-nga, Suratthani, and Nakonsitammarat. They had trapped the flies at 200m around the patients’ houses by CDC light traps. The traps had also been placed in other possible habitats such as cattle corrals, pig sites, stacks of leaves etc. These flies were then identified, however, their role in the transmission of Leishmania parasites was not confirmed. One of the important vectors of leishmaniasis of genus Phlebotomus, Phlebotomus argentipes, was also detected. They suspected them as potential vectors because they were found in the infected areas (Sukra et al. 2013).
Leishmaniasis cases in Thailand constituted only imported cases before 1999. The recent studies emphasizing indigenous leishmaniasis identify two new species, L. siamnesis and L. martiniquensis as autochthonous species among Thai patients. As reported by Chusriet al., 2014 and Kanjanopas et al., 2013, S. (Neophlebotomus) gemmea and S. (Parrotomyia) barraudi as could serve as potential vectors for L. martiniquensis (Leelayoova et al., 2017).
Srisuton et al. collected sand flies from endemic areas (Songkhla and Phatthalung Provinces) and non-endemic area (Chumphon Province) of leishmaniasis in Thailand. Head and genitalia dissection of pre-identified female sandflies were done for morphology identification, and the remaining parts were used to detect Leishmania and Trypanosoma DNA. One new vector identified as S. khawi was found to carry Leishmaniaand Trypanosoma parasites. The vector species was confirmed as a potential natural vector capable of transmitting L. martiniquensis in the human population (Srisuton et al., 2019).
References:
Apiwathnasorn C, Samung Y, Prummongkol S, Phayakaphon A, and Panasopolkul C. Cavernicolous species of phlebotomine sand flies from Kanchanaburi province, with an updated species list for Thailand. Southeast Asian J Trop Med Public Health. 2011; 42 (6): 1405-1409.
Chusri S, Thammapalo S, Silpapojakul K, Siriyasatien P.Animal reservoirs and potential vectors of Leishmania siamensis in southern Thailand. Southeast Asian J Trop Med Public Health. 2014; 45 (1): 13-19.
Kanjanopas K, Siripattanapipong S, Ninsaeng U, Hitakarun A, Jitkaew S, Kaewtaphaya P, et al. Sergentomyia (Neophlebotomus) gemmea, a potential vector of Leishmania siamensis in southern Thailand. BMC Infectious Diseases.; 2013 13: 333.
Leelayoova S, Siripattanapipong S, Manomat J, Piyaraj P, Tan-ariya P, Bualert L, et al. Leishmaniasis in Thailand: A Review of Causative Agents and Situations. Am J Trop Med Hyg.2017;96 (3): 534–542. doi:10.4269/ajtmh.16-0604.
Srisuton P, Phumee A, Sunantaraporn S, Boonserm R, Sor-suwan S, Brownell N, et al. Detection of Leishmania and Trypanosoma DNA in Field-Caught Sand Flies from Endemic and Non-Endemic Areas of Leishmaniasis in Southern Thailand. Insects. 2019; 10: 238; doi:10.3390/insects10080238.
Sukra K, Kanjanopas K, Amsakul S, Rittaton V, Mungthin M, Leelayoova S. A survey of sandflies in the affected areas of leishmaniasis, southern Thailand. Parasitol Res. 2013; 112: 297–302. DOI 10.1007/s00436-012-3137-x.
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